Bandwidth-limited and long pulse master oscillator power oscillator laser systems

ABSTRACT

Laser systems have a line-narrowed master oscillator and a power oscillator for amplifying the output of the master oscillator. The power oscillator includes optical arrangements for limiting the bandwidth of radiation that can be amplified. The limited amplification bandwidth of the power oscillator is relatively broad compared to that of the output of the master oscillator, but narrower than would be the case without the bandwidth limiting arrangements. The bandwidth narrowing arrangements of the power oscillator function primarily to restrict the bandwidth of amplified spontaneous emission generated by the power oscillator.

PRIORITY

This application is a continuation of U.S. patent application Ser. No.11/142,648, filed Jun. 1, 2005, now U.S. Pat. No. 7,418,022 which inturn claims priority from U.S. Provisional Application Ser. No.60/586,768, filed Jul. 9, 2004, the disclosures of which areincorporated herein in their entirety by reference.

TECHNICAL FIELD OF THE INVENTION

The present invention relates generally to master oscillator, poweroscillator (MOPO) laser systems. The invention relates in particular tomolecular fluorine (F₂) or excimer MOPO laser systems includingarrangements for spectral narrowing, long pulse generation, extendingcomponent lifetime, or reduction of amplified spontaneous emission(ASE).

DISCUSSION OF BACKGROUND ART

Excimer lasers including MOPO systems and master oscillator poweramplifier (MOPA) systems are extensively used as light sources for UVmicrolithography in the manufacture of state-of-the-art semiconductorintegrated circuits. Such excimer lasers have to satisfy challenging andoften mutually conflicting technical requirements, due to anever-increasing demand for smaller critical dimensions of the integratedcircuits, together with higher production throughput and reduced runningcost of UV microlithography systems. Important requirements of anexcimer laser for UV microlithography include high output power andpulse energy, long output pulse duration, narrow spectral bandwidth andhigh spectral purity of the output beam, and high reliability and longlifetime of components.

One reason for the long pulse requirement is that optical damage inducedin the projection lens of a UV microlithography system (step-and-repeatsystem or “stepper”), depends on the peak intensity of laser light inlight-pulses delivered by the excimer laser. Reducing peak intensitywhile still maintaining average power in the pulses can extend thelifetime of a projection lens without reducing system throughput. Sinceaverage power, pulse repetition rate and beam size in the projectionlens are properties of the laser and can not be changed easily, peakintensity is typically reduced by extending the duration of a laserpulse after it is delivered by the laser, and before it is delivered tothe projection lens. This is accomplished through the use of what isknown in the art as a passive pulse-stretcher, the pulse stretcher beinglocated at the laser output.

Such a passive pulse-stretcher typically consists of a beam splitter andseveral mirrors for beam folding and imaging. The mirrors form anoptical delay line. Some of the light in a pulse input to the pulsestretcher is transmitted through the beam splitter, and some of thelight is reflected. The reflected light is sent through the delay lineand, after one round trip through the delay line, returns to the beamsplitter. There, some of the light is transmitted, while some of thelight is reflected and sent through the delay line again. Such a pulsestretcher consequently generates a train of pulses, each thereof havinga fraction of the energy of the originally input pulse, and which areseparated temporally by the round trip time of the delay line.

One limitation the passive pulse-stretcher is that it is energyinefficient. This is due to scatter and absorption losses at thebeamsplitter and components of the optical delay line. Optical lossescan be as high as ten percent per round trip or even greater. Anotherlimitation is that the stretching ratio of the pulse is limited. This isbecause the energy of pulses in the train generated by thepulse-stretcher becomes vanishingly smaller after the second pulse. Thislimits the stretching factor of the pulse-stretcher to less than 3. In acase where the pulse has to be stretched by a greater factor, two ormore consecutive pulse-stretchers have to be used, which adds to energyinefficiency. Energy inefficiency converts to higher cost of the systemand higher operating cost of the system. For these reasons, it would bepreferable that the laser initially emitted a longer pulse, so that thepulse stretcher could be made simpler and more efficient.

High spectral purity, or low ASE level in the output of the laser, areimportant in order to achieve high contrast of the mask image at thewafer. This allows for reduced critical dimensions and increased processlatitude. Typically, an integral ASE level of below several times 10⁻⁴is required in systems operating at a wavelength of 193 nm.

High output power is important for high throughput of the manufacturingprocess. Typically, the required power is set by the transmission of theoptical lithography projection apparatus and required rate of waferthroughput of the stepper. A conventional, single-oscillator, excimerlaser cannot provide the required high output power with the requirednarrow spectral emission bandwidth. These two requirements are inconflict with each other, since for obtaining the narrow bandwidth theoscillator must run at low output power. At higher powers, sophisticatedmeans for spectral line narrowing including prisms and gratings cannotbe employed. Accordingly, preferred laser systems employ above-mentionedMOPA or MOPO laser systems. Such systems include a combination of alow-power master oscillator (MO) with superior narrow spectral emissionbandwidth, and a power amplifier (PA) or a power oscillator (PO), whichthen amplifies a narrow bandwidth signal from the master oscillator to ahigh power output beam.

FIGS. 1A and 1B schematically illustrate prior-art MOPA laser systems 20and 22 respectively. MOPA 20 includes a master oscillator 24 and a poweramplifier 26. Master oscillator 24 includes a line-narrowing unit 25including wavelength selective optical elements such as prisms, andoften a grating used in cooperation with prisms. As such line-narrowingarrangements in a master oscillator are well known in the art, thearrangements are not discussed in detail herein. An output beam (pulse)28 from oscillator 20 is directed by mirrors 30 and 32 into poweramplifier 26. The beam makes a single pass through amplifier 26, isamplified therein, and is output therefrom as amplified radiation. InMOPA 22, a power amplifier 27 is arranged for double pass amplification.An output beam (pulse) 28 from oscillator 20 is directed by mirrors 30and 32 into power amplifier 26. Beam 28 makes a first pass throughamplifier 27 and is reflected from a mirror 31 back through the poweramplifier. The beam is amplified on each pass through the amplifier andthe amplified beam is directed out of MOPA 22 by mirror 33.

FIGS. 2A and 2B schematically illustrate prior-art MOPO laser systems 34and 40 respectively. MOPO 34 includes a master oscillator 24 (includingline-narrowing elements 25) and a power oscillator 36 having a stableresonator formed by a partially transparent input mirror 38 and apartially transmitting outcoupling mirror 39. An output pulse 28 frommaster oscillator 24 is directed by mirrors 30 and 32 through partiallytransmitting mirror 38 into the power oscillator and seeds theoscillator. This causes the power oscillator to oscillate with thecharacteristics of the seed pulse. MOPO 40 utilizes a Cassegrain-typeunstable resonator power oscillator 37 formed between a concave mirror41 and a convex outcoupling mirror 43. Mirror 41 has an aperture 42therein which allows efficient incoupling of the seed beam (pulse) 28into the power oscillator. In this type of power oscillator, mirrors 41and 43 are usually arranged as close to the gain chamber of theoscillator (not explicitly shown) as possible. This results in a shortround-trip time, for example about 10 nanoseconds (ns), which helps toachieve maximum energy extraction efficiency from the gain chamber, butthe resulting output pulse length is only 20 ns. Having a shortestpossible resonator is consistent with the limitations of space availablewithin the volume and footprint of the laser enclosure, as both are veryvaluable in a clean room environment.

The output power of a MOPA depends directly on the power of the masteroscillator. The optical design of a MOPA is simpler than that of a MOPO,at least when the power amplifier is used in single pass as depicted inFIG. 1A. However, the power extraction efficiency is limited, especiallyif the master oscillator emits low power. For a multiple pass amplifier,such as illustrated in FIG. 1B, the setup quickly becomes complicated,as the output and input beams have to be spatially separated. Since theoutput power of the amplifier depends on the input from the masteroscillator, the master oscillator still has to emit comparably highpower, which sets a limit to narrowing of the spectral emissionbandwidth in the master oscillator.

In a MOPO arrangement, the master oscillator functions only as a lowpower seed source for the power oscillator, which oscillates and emitshigh power with the same bandwidth as the master oscillator. The poweroscillator is a fully functional oscillator, which, in free lasing,emits maximum output power stored in the gain medium. The power of theseed source (master oscillator) has limited influence on the outputpower from the power oscillator. Since only low power is needed forseeding, the master oscillator can run at a much lower output powerlevel. Because of this sophisticated methods for narrowing the spectralemission bandwidth can be employed. A MOPO arrangement, accordingly, isstrongly preferred for high power narrow bandwidth operation.

In a MOPO system, however, degradation of internal optical componentsdue to exposure to high energy UV radiation occurs more rapidly than ina MOPA laser. This is especially true of the outcoupling mirror of aMOPO laser, which is at normal incidence to a high-intensity outputbeam. Further, the power oscillator of a MOPO laser, if not seeded,oscillates independently and emits light with broad spectralcharacteristics. Also, if seeding is insufficient, a background of broademission characteristics will overlay the narrow bandwidth spectrum ofthe seed source. Such background is highly undesirable, especially forlithography applications. Even when seeding is nearly perfect, somebackground of amplified spontaneous emission (ASE) still exists. Suchbackground has to be suppressed to obtain high spectral purity.

Another disadvantage of a MOPO laser system results from cross talkbetween the master oscillator and the power amplifier. This cross talkcan degrade the performance of the master oscillator. Seeding of thepower amplifier is usually accomplished by means of a partiallyreflecting mirror, which is also the rear mirror of the poweroscillator, as discussed above with respect to MOPO 34 of FIG. 2A. Boththe seed beam and the rear mirror have to be aligned to the axis of thepower oscillator. The master oscillator and power oscillator share thesame optical axis, and the rear mirrors and outcoupling mirrors of bothoscillators are normal to the optical axis. Cross talk results frommutual feed back between the two oscillators.

Two solutions have been applied to this cross talk problem. One solutionis to provide sufficient optical length between the two oscillators thatthe entire length of the seed pulse is already emitted from the masteroscillator before the cross talk can set in. Providing an optical delayline of sufficient length between master oscillator and power oscillatorcan effectively suppress cross talk. Such a delay line, however,consumes valuable space inside a laser enclosure.

Another potential solution would be to align the mirrors of the poweroscillator off the optical axis of the master oscillator. This option,unfortunately, is not feasible for a linear power oscillator. There is aneed to overcome the cross-talk problem and other above-discussedshortcomings of MOPO lasers.

SUMMARY OF THE INVENTION

In one aspect, the present invention is directed to a laser systemcomprising a master oscillator for generating a beam of laser radiationand a power oscillator for amplifying the laser radiation beam, anddelivering the amplified laser pulse. The power oscillator has aresonator including least one optical arrangement for restricting thebandwidth of radiation of radiation that can be amplified therein.

A preferred bandwidth-restricting element is a prism. Two prisms may beincluded in the power oscillator and arranged to fold the resonator axisat 90°.

In another aspect of the present invention the resonator of the poweroscillator is a ring resonator and the beam of radiation is in the formof a laser pulse. The ring resonator provides for an extended time inthe resonator between amplifying events, compared with a linearresonator having the same gain medium and the same round trip distance.The extended time between amplification events provides for a longerpulse length than would be obtain in a power oscillator having a linearresonator having the same round trip distance. The extended distancebetween amplification events also provides for reducing ASE.

In yet another, non-exhaustive aspect of the present invention, themaster oscillator includes a linear laser-resonator having a length ofabout 2 meters or greater. This provides for a greater time betweenamplification events compared with that of a shorter linear resonatorhaving a length less than 2 meters. Advantages of the extended timebetween amplifying events include those discussed above with referenceto the ring resonator.

The long resonator affords an option of including a grazing incidencereflective telescope therein. The grazing incidence telescope isarranged to expand a light beam circulating in the resonator such thatthe expanded beam is incident on an outcoupling mirror of the resonator.Expanding the beam on the outcoupling mirror reduces power density on,and consequently the possibility of optical damage to, the outcouplingmirror. Light being incident at grazing incidence in the telescope,power density on mirrors of the telescope is also reduced.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute apart of the specification, schematically illustrate a preferredembodiment of the present invention, and together with the generaldescription given above and the detailed description of the preferredembodiment given below, serve to explain the principles of the presentinvention.

FIG. 1A schematically illustrates a prior-art MOPA laser systemincluding a line-narrowed master oscillator, the output of which isamplified by a single-pass power amplifier.

FIG. 1B schematically illustrates a prior-art MOPA laser systemincluding a line-narrowed master oscillator, the output of which isamplified by a double-pass power amplifier.

FIG. 2A schematically illustrates a prior-art MOPO laser systemincluding a line-narrowed master oscillator, seeding a power oscillatorhaving a stable resonator configuration.

FIG. 2B schematically illustrates a prior-art MOPO laser systemincluding a line-narrowed master oscillator, seeding a power oscillatorhaving a Cassegrain type unstable resonator configuration.

FIG. 3 schematically illustrates a first embodiment of a MOPO lasersystem in accordance with the present invention including aline-narrowed master oscillator, seeding a bandwidth-limited poweroscillator having a stable linear resonator formed between a partiallytransmissive rear mirror and a partially transmissive outcouplingmirror, a gain medium in the resonator, and a pair of triangular prismslocated between the gain medium and the rear mirror, with an opticalarrangement being provided for injecting a seed pulse into the poweroscillator via the rear mirror thereof.

FIG. 4 schematically illustrates a second embodiment of a MOPO lasersystem in accordance with the present invention including aline-narrowed master oscillator, seeding a bandwidth-limited poweroscillator having a stable linear resonator formed between a partiallytransmissive rear mirror and a partially transmissive outcouplingmirror, a gain medium in the resonator, one triangular prism locatedbetween the gain medium and the rear mirror, and another triangularprism located between the gain medium and the outcoupling mirror, withan optical arrangement being provided for injecting a seed pulse intothe power oscillator via the rear mirror thereof.

FIG. 5 schematically illustrates a third embodiment of a MOPO lasersystem in accordance with the present invention including aline-narrowed master oscillator, seeding a bandwidth-limited poweroscillator having a stable linear resonator formed between a partiallytransmissive rear mirror and a partially transmissive outcouplingmirror, a gain medium in the resonator, one right-angle triangular prismlocated between the gain medium and the rear mirror, and anotherright-angle triangular prism located between the gain medium and theoutcoupling mirror, with an optical arrangement being provided forinjecting a seed pulse into the power oscillator via the rear mirrorthereof.

FIG. 6 schematically illustrates a fourth embodiment of a MOPO lasersystem in accordance with the present invention, similar to the laser ofFIG. 3, but wherein the seed pulse is injected into the power oscillatorvia the outcoupling mirror thereof.

FIG. 7 schematically illustrates a fifth embodiment of a MOPO lasersystem in accordance with the present invention, similar to the laser ofFIG. 4, but wherein the seed pulse is injected into the power oscillatorvia the outcoupling mirror thereof.

FIG. 8 schematically illustrates a sixth embodiment of a MOPO lasersystem in accordance with the present invention, similar to the laser ofFIG. 5, but wherein the seed pulse is injected into the power oscillatorvia the outcoupling mirror thereof.

FIG. 9 schematically illustrates a seventh embodiment of a MOPO lasersystem in accordance with the present invention including aline-narrowed master oscillator, seeding a bandwidth-limited poweroscillator having a ring resonator formed between a partiallytransmissive mirror and two fully transmissive mirrors, a gain medium inthe resonator, and a pair of triangular prisms located between the gainmedium and the partially transmissive mirror, with an opticalarrangement being provided for injecting a seed pulse into the poweroscillator via the partially transmissive mirror thereof.

FIG. 10 schematically illustrates an eighth embodiment of a MOPO lasersystem in accordance with the present invention, similar to the laser ofFIG. 9 but wherein a seed pulse is injected into the power oscillatorvia reflection from one face of an optical wedge prism located in theresonator.

FIG. 11 schematically illustrates a simple triangular prism configuredto compress a beam normally incident on one face thereof, while allowingthe compressed beam to exit an adjacent face of the prism at Brewster'sangle thereto.

FIG. 12 schematically illustrates a Pellin-Broca prism configured toturn a beam incident thereon through 90° by total internal reflection,with the beam entering the prism at an entrance face thereof atBrewster's angle thereto and exiting the prism at an exit face thereofat Brewster's angle thereto, and with the beam having the same width onentering and exiting the prism.

FIG. 12A schematically illustrates the Pellin-Broca prism of FIG. 12rotated such that the beam enters the prism at and angle less thanBrewster's angle to the entrance face and exits the prism at an anglegreater than Brewster's angle to the exit face with the beam beingcompressed as a result of traversing the prism and being turned by anangle greater than 90°.

FIG. 13 schematically illustrates a ninth embodiment of a MOPO lasersystem in accordance with the present invention including aline-narrowed master oscillator, seeding a bandwidth-limited poweroscillator having a ring resonator formed by one partially transmissivemirror, two fully reflective mirrors, two Pellin-Broca prisms innon-Brewster alignment, and a triangular prism having one face thereofserving as a grazing incidence outcoupling mirror, with an opticalarrangement being provided for injecting a seed pulse into the poweroscillator via the partially transmissive mirror thereof.

FIG. 14 schematically illustrates a tenth embodiment of a MOPO lasersystem in accordance with the present invention including aline-narrowed master oscillator, seeding a bandwidth-limited poweroscillator having a ring resonator formed by one partially transmissiveoutcoupling mirror, one fully reflective mirror, and two Pellin-Brocaprisms in Brewster angle alignment, with an optical arrangement beingprovided for injecting a seed pulse into the power oscillator viareflection from one face an optical wedge prism located in theresonator.

FIG. 15 schematically illustrates principles of long pulse generation ina long, stable, standing-wave linear resonator for a power oscillator inaccordance with the present invention.

FIG. 16 schematically illustrates principles of long pulse generation ina ring resonator for a power oscillator in accordance with the presentinvention.

FIGS. 17A and 17B are views seen in mutually perpendicular axesschematically illustrating an eleventh embodiment of a MOPO laser inaccordance with the present invention including a power oscillatorhaving a stable, long, linear resonator in accordance with theprinciples of FIG. 15, the resonator including a grazing incidencetelescope for providing an expanded output beam and including anoptional spectral bandwidth limiting element, and wherein a seed beam isinjected into the resonator via a rear mirror thereof.

FIGS. 18A and 18B are views seen in mutually perpendicular axesschematically illustrating details of beam expansion by the grazingincidence telescope of FIGS. 17A-B.

FIGS. 19A and 19B are views, seen in mutually perpendicular axes,schematically illustrating a twelfth embodiment of a MOPO laser inaccordance with the present invention similar to the laser of FIGS.17A-B but wherein the grazing incidence, beam expanding telescope isadditionally configured to fold the resonator axis of the masteroscillator by 90°.

FIG. 20 schematically illustrates a thirteenth embodiment of a MOPOlaser in accordance with the present invention similar to the laser ofFIGS. 17A-B but wherein the seed beam is injected into the resonator viagrazing incidence reflection from one surface of a prism having athin-film mirror on another surface thereof.

FIG. 21 schematically illustrates a fourteenth embodiment of a MOPOlaser in accordance with the present invention similar to the laser ofFIGS. 19A-B but wherein the seed beam is injected into the resonator viagrazing incidence reflection from the surface of a prism having a thinfilm mirror on another surface thereof.

FIGS. 22A and 22B are views, seen in mutually perpendicular axes,schematically illustrating the grazing incidence beam-expandingtelescope of FIGS. 17A-B used to expand an output beam delivered by aMOPO/MOPA laser.

FIG. 23 schematically illustrates a fifteenth embodiment of a MOPO laserin accordance with the present invention, similar to the ring-resonatorMOPO laser of FIG. 13, but wherein the triangular prism, having one facethereof serving as a grazing incidence outcoupling mirror, is located ina gain chamber of the power oscillator, and output radiation leaves thegain chamber via a Brewster angle window located therein.

FIG. 24 schematically illustrates one alternative arrangement of thegain chamber of FIG. 23 located in a generic linear resonator.

DETAILED DESCRIPTION OF THE INVENTION

As one solution to the above-discussed problem of ASE suppression andachieving high spectral purity in a MOPO laser, certain embodiments ofMOPO lasers in accordance with the present invention have a poweroscillator including optical elements for limiting spectral bandwidththereof. The emission spectrum of the inventive bandwidth-limited poweroscillators, in a free lasing mode, is still relatively broad, comparedto a line-narrowed master oscillator, but narrow compared to the nonbandwidth-limited power oscillators of prior-art MOPO lasers. Thebandwidth of the main lasing line of the power oscillator, whenamplifying a seed beam, is determined primarily by the line-narrowingelements of the master oscillator. The bandwidth-narrowing elements ofthe inventive power oscillator function primarily to restrict thebandwidth of ASE.

By way of example, if the power oscillator is bandwidth-limited to about10% of what the bandwidth would be without bandwidth-limiting elements,then the background of ASE in the seeded bandwidth-limited poweroscillator is expected to be reduced by the same factor. For 90% of theASE bandwidth, the power oscillator acts as a low-gain amplifier, sincemost of the ASE light is diverted by the bandwidth-limiting elementsaway from the gain-providing medium of the power oscillator. Incontrast, the seeded light is only slightly affected, since thebandwidth-limiting elements of power oscillator are tuned to thewavelength of the master oscillator, and, accordingly, the poweroscillator always has high-gain for the seed signal (seed pulse or seedbeam).

Referring again to the drawings, wherein like features are designated bylike reference numerals, FIG. 3 schematically illustrates a firstembodiment 50 of a MOPO laser in accordance with the present invention.MOPO 50 includes a master oscillator 24 (including line-narrowingelements 25) and bandwidth-limited power oscillator (BLPO) 52. BLPO 52has a resonator 54 formed between a partially transmissive rear mirror56 and a partially transmissive outcoupling mirror 58. Included inresonator 54 is a gain chamber 60 including lasing gases and parallel,spaced-apart planar electrodes, across which an electrical pulse can bedelivered to create a discharge in, and thereby energize, lasing gas(gain-medium) therebetween for providing optical gain. Only a lower oneof the electrodes 65B is depicted in FIG. 3.

It should be noted, here, that FIG. 3 is a view seen along what istermed by practitioners of the excimer laser art as the “long beam-axis”of resonator 54. An excimer laser typically generates a beam that iswider than it is high. The long beam axis (perpendicular to the plane ofthe electrodes, i.e., perpendicular to the plane of the drawing) is theaxis in which the beam in gain chamber 60 is widest. It should also benoted that in FIG. 3, and other drawings herein, the spatial location ofthe master oscillator with respect to the power oscillator is chosenprimarily for convenience of illustration. In practice the masteroscillator may be in some other location relative to the poweroscillator without departing from the spirit and scope of the presentinvention.

Two isosceles triangular prisms 62 are located in resonator 54 betweenthe gain chamber (gain-medium) and rear mirror 56. Here, the triangularprisms are configured and arranged to cause the path 64 of laser lightcirculating in resonator 54 to be turned a total of 90° between the gainmedium and the mirror. It should be noted that in FIG. 3, and all otherdrawings of the invention referred to herein, the path of a light beamis generally indicated by a single line with open arrowheads indicatingthe direction of travel. Where a beam width is explicitly depicted, twolines are used to indicate bounds of the beam.

Continuing with reference to FIG. 3, there is no requirement in MOPO 50that the beam path be turned exactly at 90°. Preferably, prisms 62 areconfigured and arranged such that light is incident on faces 62A and 62Bof each prism at or close to the Brewster angle. This avoids requiringanti-reflection coatings on the prisms. Seed beam 28 from masteroscillator 24 is reflected from mirror 30 and delivered to BLPO 52through mirror 56 thereof. The seed beam is amplified by BLPO 52 anddelivered as output radiation through outcoupling mirror 58.

The bandwidth limiting arrangement of prisms 62 is tuned to the samewavelength as the master oscillator. Accordingly, losses in poweroscillator are minimal for the seed light. Light having wavelengthsother than the wavelength of the seed light is diverted away from theresonant path 64 of the seed light. As most of the ASE has suchwavelengths, most of the ASE is so diverted and resonator losses for ASEare much higher than those for the seed light, resulting in a lower gainfor the ASE. This results in a corresponding reduction in ASE as apercentage of amplified seed light (output).

Further, each surface of prisms 62 acts as a partial polarizer. At eachof the surfaces p-polarized light is transmitted with relatively lowlosses, for example less than about 1%, while s-polarized lightencounters significant losses, for example, greater than about 10%. Witheach pass through such a surface the s-polarized portion of the beam isfurther attenuated. Inside the resonator, the laser beam is p-polarizedalmost perfectly. In contrast, ASE is mainly randomly polarized. Becausethe s-polarized portion of the ASE encounters high losses at the prismsurfaces, ASE is further attenuated. Dispersion and polarization by theprisms, together, reject most of the ASE from the output of the poweroscillator.

Losses per round trip for signal light in the BLPO are much smaller thanthose for the master oscillator, due to a larger acceptance bandwidthand minimum number of optical components in the BLPO. Accordingly, noactive means for adjusting tuning of the BLPO wavelength is needed,resulting in high output power and low sensitivity to misalignment.

The timing of a gain-medium-energizing discharge in master oscillator 24with respect to the timing of a gain-medium-energizing discharge in gainchamber 62 of BLPO 52 is adjusted for optimum seeding efficiency withthe lowest possible seed power. Since the seed signal has to competewith the spontaneous emission within the gain medium of the BLPO, thetiming is adjusted so seeding takes place just before the natural pulseevolution (free lasing oscillation) inside the BLPO can start. Becauseof this, most of the power of the gain medium is extracted before mostof the spontaneous oscillations have a chance to develop.

If the seed pulse were to be delivered too early, the gain in gainchamber 60 of the BLPO 52 would not be sufficiently evolved, and thepower of the seed signal would decay before oscillation could start. Ifthe seed signal were to arrive too late, the signal would compete with astrongly evolving (free lasing) ASE pulse. Given these effects oftiming, the ratio of power within and outside the desired bandwidth inthe BLPO output will depend on the relative timing of the seed pulsedischarge and BLPO discharge, in addition to the bandwidth limiting (ASEdiverging) effect of prisms 62. Accordingly, this timing must beoptimized in order to take maximum advantage of the ASE-reducing effectof the inventive bandwidth-limited power oscillator.

FIG. 4 is a long beam-axis view schematically illustrating a secondembodiment 66 of a MOPO laser in accordance with the present invention.MOPO 66 includes a master oscillator 24 (including line-narrowingelements 25) and bandwidth-limited power oscillator (BLPO) 68. BLPO 68has a resonator 70 formed between a partially transmissive rear mirror56 and a partially transmissive outcoupling mirror 58. Included inresonator 70 is a gain chamber 60, as discussed above. One isoscelestriangular prism 62 is located in resonator 70 between the gain chamber(gain-medium) and rear mirror 56. Another triangular prism 62 is locatedin resonator 70 between the gain chamber (gain-medium) and outcouplingmirror 58. Preferably, again, the prisms are configured and arrangedsuch that light is incident on isosceles faces 62A and 62B of each prismat or close to the Brewster angle to avoid requiring anti-reflectioncoatings on the prisms. Seed beam 28 from master oscillator 24 isreflected from mirrors 30 and 32 and delivered to BLPO 68 through mirror56 thereof. The seed beam is amplified by BLPO 68 and delivered asoutput radiation through outcoupling mirror 58. Arranging one prism ateach end of the gain chamber as opposed arranging both prisms at one endof the gain chamber has an advantage that the resonator is relativelyinsensitive to environmentally induced changes in alignment of the gainchamber.

The arrangement of MOPO 66 has an advantage that prisms 62 may serve aswindows for gain chamber 60 replacing the standard Brewster windows (notshown). This would reduce the number of optical components in resonator70, and, accordingly, reduce resonator losses.

FIG. 5 is a long beam-axis view schematically illustrating a thirdembodiment 72 of a MOPO laser in accordance with the present invention.MOPO 72 includes a master oscillator 24 (including line-narrowingelements 25) and bandwidth-limited power oscillator (BLPO) 74. BLPO 74has a resonator 76 formed between a partially transmissive rear mirror56 and a partially transmissive outcoupling mirror 58. Included inresonator 76 is a gain chamber 60. Resonator 76 includes tworight-angled triangular prisms 63. One of the right-angled prisms 63 islocated in resonator 76 between the gain chamber (gain-medium) and rearmirror 56. Another of the right-angled prisms 63 is located in resonator76 between the gain chamber (gain-medium) and outcoupling mirror 58.Seed beam 28 from master oscillator 24 is reflected from mirrors 30 and32 and delivered to BLPO 74 through mirror 56 thereof. The seed beam isamplified by BLPO 74 and delivered as output radiation throughoutcoupling mirror 58.

The arrangement of MOPO 72 also has an advantage that prisms, hereright-angled prisms 63, may serve as windows for gain chamber 60replacing the standard windows (not shown). An additional advantage ofthe arrangement is that the right-angled prisms can be arranged as beamexpanders. This is discussed in detail further hereinbelow. Thisincreases the area of a circulating beam on mirrors 56 and 58, therebyreducing light intensity on the mirrors and reducing the possibility ofoptical damage to the mirrors.

In all above-discussed embodiments of the inventive MOPO laser, the BLPOis seeded by directing a seed pulse from master oscillator 24 into thebandwidth-limited power oscillator through rear mirror 56 thereof. Thisshould not be construed as limiting the inventive MOPO lasers to thatmethod of seeding. The inventive MOPO lasers may also be seeded bydirecting a seed beam (pulse) 28 into the BLPO via outcoupling mirror 58thereof. By way of example, FIG. 6 schematically illustrates a fourthembodiment 78 of a MOPO laser in accordance with the present invention.MOPO 78 is similar to MOPO 50 of FIG. 3 with exceptions as follows.Partially transmitting resonator rear mirror 56 of MOPO 50 is replacedby fully reflecting rear mirror 57. Seed beam 28 from master oscillator24 is reflected from mirror 30 and partially reflective (between about1% and 20% reflective) and partially transmissive beamsplitter 35 anddelivered to BLPO 52 through outcoupling mirror 58 thereof.

FIG. 7 and FIG. 8 are long beam-axis views schematically illustratingfifth (80) and sixth (82) embodiments of an MOPO laser in accordancewith the present invention. MOPO 80 is similar to MOPO 72 of FIG. 5.MOPO 82 is similar to MOPO 78 of FIG. 6. In each case, a fullyreflecting rear mirror 57 is substituted for partially reflecting mirror56. In each case, also, a seed beam 28 from master oscillator 24 isreflected from a mirror 30 and a partially reflective and partiallytransmissive beamsplitter 35 and delivered to the bandwidth-limitedpower oscillator through outcoupling mirror 58 thereof.

While embodiments of the inventive MOPO laser discussed above willprovide reduced ASE compared with prior-art MOPOs, these embodimentswill not provide a pulse length significantly longer than a prior-artMOPO. Set forth below is a description of embodiments of MOPO lasers inaccordance with the present invention that provide for longer pulselength than above described embodiments while still providing reducedASE compared with prior-art MOPOs, and while providing other advantagesover prior art MOPOs. These advantages include greatly reduced crosstalk between the power oscillator and the master oscillator, and theability to provide beam expansion in the BLPO for reducing opticaldamage to the BLPO components.

FIG. 9 is a long beam-axis view schematically illustrating a seventhembodiment 84 of a MOPO laser in accordance with the present invention.MOPO 84 includes a master oscillator 24 (including line-narrowingelements 25) and bandwidth-limited power oscillator (BLPO) 86. BLPO 86has a traveling-wave resonator (ring-resonator) 88 formed by a partiallytransmissive mirror 90, a fully reflective mirror 92 and another fullyreflective mirror 94. Included in resonator 88 is a gain chamber 60, asdescribed above. Resonator 88 includes isosceles triangular prisms 62cooperatively arranged to turn the beam path 64 in the resonator through90°. These prisms have the bandwidth limiting function described abovewith respect to other embodiments of the inventive MOPO. Seed beam 28from master oscillator 24 is reflected from mirror 30 and delivered intoresonator 88 through mirror 90 thereof. The seed beam is amplified byBLPO 86 and delivered as output radiation through mirror 90. If thetransmission requirements for mirror 90 to allow seed beam injection andoutcoupling should be in conflict, a partially transmissive mirror maybe substituted for either of mirrors 92 and 94 and used as anoutcoupling mirror, albeit at the expense of increased resonator loss.

FIG. 10 is a long beam-axis view schematically illustrating an eighthembodiment 96 of a MOPO laser in accordance with the present invention.MOPO 96 is similar to MOPO 84 of FIG. 9 except for the manner in whichseed beam 28 is coupled into resonator 88 of BLPO 86. In MOPO 96, seedbeam 28 from master oscillator 24 is reflected by mirrors 30 and 37 ontoface 98B of the prism 98 nearest to mirror 91. The angle of incidence ofbeam 28 on face 98B is selected such that the reflected fraction of theseed beam follows beam path 64 in resonator 88.

In above-discussed, linear resonator embodiments of the inventivebandwidth-limited power oscillator, a beam passes through gain chamber60 twice within one round trip and the resonator mirrors 56 and 58 arerelatively close to the gain chamber. Because of this, there are onlyvery short time periods in which the beam is not traversing the gainchamber. Consequently, power stored in the gain medium is extractedrapidly therefrom within a few passes through the gain chamber. Thisresults in short pulse lengths of about 20 nanoseconds (ns), comparableto the pulse lengths in prior-art MOPO lasers.

In a ring resonator such as resonator 88, the time between successivepasses of a pulse through gain chamber 60 is much longer than in theabove discussed standing-wave resonators, since the beam has to befolded around the gain chamber. In contrast to any linear resonator, thebeam passes only once through the gain medium per round trip.Consequently, more round trips are required to extract the power fromthe gain medium, and the resulting pulse length is automatically longer.One limitation, which sets an upper limit for the round trip time, isthat the lifetime of the optical gain is not indefinite. If the roundtrip time becomes too long, the gain may already have decayed when apulse arrives at the gain-medium following a previous round trip. Ifthat were the case, the pulse would not be amplified further.Accordingly power extraction efficiency would decrease and pulse powerwould decrease. It has been determined, however, that a round trip time(resonator round trip path length) can be chosen that is a goodcompromise between the output pulse power and the pulse length. A pulselength in excess of 30 ns can be achieved.

The divergence of ASE is higher than that of the laser beam circulatingin resonator 88. This causes the ASE beam size to expand much fasterthan the main laser beam. In principle, the ASE can be suppressed simplyby propagating the laser beam then passing the laser beam through anaperture. The aperture filters out the ASE of high divergence. An effectof this kind takes place inside of the resonator. The longer thedistance the amplified light has to travel before it encounters the gainmedium again, the more the ASE is expanded. The gain medium acts as anaperture, and only a smaller portion of the ASE is amplified. In a ringresonator, this effect is much greater than in a linear resonator ofcorresponding round trip length. Reduction of ASE through misalignmentor diversion of the ASE by the prisms is also greater in the ringresonator than in a linear resonator of corresponding round trip length.

Laser light circulating in ring resonator 88 has a natural divergenceresulting from diffraction effects. A result of this is that the size ofthe beam expands with distance, absent any measures to prevent thisexpansion. Ring resonator 88 functions optimally only if, after oneround trip in the resonator, the beam size is well matched to theoriginal size of the beam. If this is not the case, the beam canencounter significant losses resulting in a decrease in efficiency. Theeffect of this natural divergence should preferably be compensated. Onemeans of effecting such compensation is to compress the beam using oneor more prisms. A description of the mechanism of compressing a beamwith a prism is set forth below with reference to FIG. 11, and FIGS. 12Aand 12B.

FIG. 11 schematically illustrates a prism 100 used as a beam compressor.A beam 102, linearly polarized in the plane of the drawing (indicated byarrow P) and having a width W₀, enters the prism normally to inputsurface 100A of the prism. At output surface 100B of the prism, the beamis refracted and the refracted beam 102R has W_(R), which is smallerthan W₀. Usually, such a prism would have an anti-reflective coating(not shown) on input surface 100A. If the angle between faces 100A and100B is made equal to ninety degrees minus the Brewster angle(90°-θ_(B)) output surface 100B does not need to be coated as beam 102Ris p-polarized with respect to that surface and exits the surface at theBrewster angle, at which the reflection for p-polarized light is zero.The Brewster angle for a calcium fluoride prism at an ArF excimer laserwavelength of 193 nanometers (nm) is about 56.3° nm.

For the prism arrangement depicted in FIG. 11, beam 102R is compressedto about 60% of its original width W₀. The compression ratio can bechanged by slightly rotating the prism. However, some of the p-polarizedlight will then be reflected off the exit surface and beam will beslightly attenuated. The stronger the compression and the larger theexit angle, the greater the attenuation will be. For high compressionratios higher than 60%, it is preferable to arrange two or more prismsin series with each having the preferred (Brewster's) exit angle.

Instead of the simple prism beam-expander of FIG. 11, it is preferableto use one or more Pellin-Broca prisms for beam expansion. APellin-Broca prism is a prism that combines the function of dispersionprism and a 90°-folding prism or turning prism in a single opticalelement. Such a prism can be used advantageously in a ring-resonatorpower oscillator in accordance with the present invention to provide acombination resonator mirror and bandwidth-limiting element.

FIG. 12 schematically illustrates a Pellin-Broca prism 104 used to turna p-polarized beam 102 through an angle of 90° through a combination ofrefraction and total internal (TIR) 90° with zero Fresnel reflectionlosses on entering and exiting the prism. Prism 104 has an entrance face104A (the short face), a TIR face 104B and an exit face 104C (the longface). Entrance face 104A is at 90° to exit face 104C, and TIR face 104Bis inclined at 45°-θ_(B) to exit face 104C.

Beam 102 is incident on an entrance face 104A of the prism at theBrewster angle θ_(B), in this example, about 56°. The beam is refractedat surface 104A at an angle of 90°-θ_(B), i.e., the Brewster angle inthe prism, to surface 104A, and is incident on and reflected from TIRface 104B at an angle of 45°. The beam is incident on exit face 104C atthe refraction angle, and, accordingly, exits face 104C as refractedbeam 102R at an angle θ_(B) (the external Brewster angle). As the beamangles on the entrance and exit surfaces are equal, width W_(R) ofrefracted beam 102R is equal to width W_(R) of the incident beam.

By rotating Pellin-Broca prism 104 such that a beam incident thereon isincident at slightly less than the Brewster angle, the prism can be usedto compress beam 102. FIG. 12A illustrates this condition, where beam102 is incident on entrance face 104A of prism 104 at the Brewster angleless some angle δ₁. This increases the angle of incidence (andreflection) of the beam on TIR face 104B and consequently on exit face104C. Beam 102R leaves exit face 104C at some angle δ₂ greater than theBrewster angle, where δ₂ is greater than δ₁. Beam 102R will be at anangle of 90°+δ₁+δ₂ to incident beam 102 and will have a width W_(R) thatis less than width W₀ of the incident beam. By way of example, if, for aprism having θ_(B)=56°, the angle of incidence of beam 102 on entranceface 104A is 52°, i.e., δ₁ is 4°, beam 102R will exit face 104C at anangle of 64.5 thereto, i.e., with δ₂ equal to 8.5°. Beam 102R will beinclined at 102.5° to beam 102 and width W_(R) will be about 70% ofwidth W₀. For a calcium fluoride prism, the reflection loss at face 104Afor light having a wavelength of 193 nm will be about 0.2% and the lossat face 104C will be about 1.0%. These losses are tolerable comparedwith losses that would be experienced in a conventional front surfaceturning mirror at the same wavelength.

It should be noted, here, that when combining two Pellin-Broca prisms ina BLPO in accordance with the present invention, the net effect ofdispersion, total beam folding angle and beam expansion depend stronglyon the orientation of the prisms with respect to each other and withrespect to circulating beam. In an arrangement, which folds the beamthrough 180°, four different relative orientations of the prisms arepossible.

In a first of these relative orientations, the beam may enter bothprisms via the short face 104A. In a second of these relativeorientations, the beam may enter both prisms via the long face 104C. Ineither of these first and second orientations, when the two prisms arearranged at 90° with respect to each other, the net dispersion is zeroand the total beam-folding angle is always 180°, independent of theangle of the incident beam. The beam exits the pair of prisms with thesame size as it entered. Such a configuration is highly desirable ifslight changes in wavelength or direction of the beam must not misalignthe BLPO. However, this configuration is not suitable for providing linenarrowing.

In a third of the relative orientations, the beam enters the first prismvia the short face, and the second prism via the long face. In a fourthof the relative orientations, the beam enters the first prism via thelong face and enters the second prism via the long face. If the prismsare oriented in these third and fourth orientations, but still at 90°with respect to each other, the total folding angle becomes wavelengthdependent. The net dispersion is not zero and the oscillator becomessensitive to the wavelength and the angle of the beam.

For any of the four orientations, if the prisms are used in some“non-Brewster” alignment to a beam, for example, similar to thatdepicted in FIG. 12A, the desired magnification ratio (beam compressionor expansion) can be chosen within certain limits. In such non-Brewsteralignment, the total beam-folding angle differs from 180°, and the netdispersion is never zero. Magnification, total folding angle, anddispersion are interdependent, so not all combinations are possible.Combinations can be found, however, by using simple trigonometry, whichgive a desired magnification for a chosen folding angle.

Pellin-Broca prisms may have one disadvantage inasmuch as the opticalpath length within a prism is quite long. If the prism material issufficiently absorbing for light traversing the prism, or if the prismmaterial degrades, then losses and thermal distortions can be expected.However, a beam inside the prism is always larger than the input oroutput beam. Because of this, especially when using the prisms in acomparably low power beam, and if the beam is expanded sufficiently,this disadvantage is outweighed by the advantages of the prism.

FIG. 13 is a long beam-axis view schematically illustrating a ninthpreferred embodiment 110 of a MOPO in accordance with the presentinvention including a master oscillator 24 having line-narrowingelements 25, and a BLPO 112 including a ring resonator 114. Ringresonator 114 includes a gain chamber 60 and two Pellin-Broca prisms104. Pellin-Broca prisms 104 serve as fold mirrors for resonator 114.Each of the Pellin-Broca prisms is arranged as a beam compressor, asdiscussed above with reference to FIG. 12A, for compensating for naturaldivergence of a laser beam circulating in the resonator. In the relative(orientation) orientation shown, with each of the prisms (prism faces)in a non-Brewster alignment with the circulating beam the prisms alsoprovide some limited dispersion as discussed above. Ring resonator 114is completed by two fully reflective fold mirrors 120 and 122, apartially transmissive fold mirror 124, and a prism 126 having a face126A thereof arranged such that light circulating in the resonator isincident at grazing incidence thereon. By grazing incidence is meant anangle of incidence greater than about 60° and preferably greater thanabout 70°. A seed beam 28 from master oscillator 24 is reflected frommirrors 30 and 32 and delivered to the bandwidth-limited poweroscillator through mirror 124 of resonator 114.

Face 126A of prism 126 serves as an outcoupling mirror of the resonator114. As the outcoupling mirror in any ring oscillator sees the highestcirculating power, it is important to provide that this component isresistant to degradation by optical damage. In ring-resonator 88 ofFIGS. 9 and 10, a conventional plane mirror 90 is used as an outcouplingmirror. The mirror is at an angle of incidence of 45° to incident light,as are other folding mirrors of the resonator. Compared to a normalincidence mirror in a straight, standing-wave resonator, the powerdensity on a 45° incidence mirror is reduced by a factor of about 1.7.Further reduction of the power density requires a larger angle ofincidence.

At incidence angles greater than the Brewster angle, the reflectivity ofan uncoated surface for p-polarized light rapidly increases withincreasing incidence angle. At an angle of 70° reflection from anuncoated CaF₂ prism is for p-polarized light is about 5% and at an angleof 80° is about 23%. In a very high gain oscillator a reflectivity inthis range is adequate for an outcoupling mirror. Using the prismarrangement of prism 126 provides an outcoupling mirror that has ahigher angle of incidence and correspondingly greater tolerance to highpower than a 45° incidence mirror, and does not require an opticalcoating. If surface 126A or prism 126 is arranged cooperative with otherelements of resonator 114 such that light is incident at an angle ofincidence of 80° on the surface, the beam at the surface is expanded bya factor of 5.75, and the reflectivity is about 23%. Inside prism 126the beam is expanded by a factor of 4.3. The prism therefore actssimultaneously as a beam expander and beam splitter. By rotating theprism, the split ratio can be conveniently adjusted. The output beamleaves the prism highly expanded, thereby reducing power density on anyoptical elements required to interact with the output beam. This allows,for an anti-reflecting coating (not shown) to be used on output surface126B of prism 126.

FIG. 14 is a long beam-axis view schematically illustrating a tenthpreferred embodiment 130 of a MOPO in accordance with the presentinvention, having a master oscillator 24 including line-narrowingelements 25, and a BLPO 132 including a ring resonator 134. Ringresonator 134 includes a gain chamber 60 and two Pellin-Broca prisms104. Pellin-Broca prisms 104 serve as fold mirrors for resonator 134. Inthis relative orientation the beams provide a relatively highdispersion. In resonator 134 each of the Pellin-Broca prisms is arrangedto turn light though 90° as discussed above with reference to FIG. 12. Abeam entering and leaving the prisms has the same width and is incidenton refractive surfaces of the prism at the Brewster angle. Accordingly,there is no beam compression. Ring resonator 114 is completed by a fullyreflective fold mirror 92, and a partially transmissive fold mirror 90that serves as an outcoupling mirror. A seed beam 28 from masteroscillator 24 is injected into resonator 134 via reflection from mirrors30 and 37 and reflection from surface 98B of a prism 98 located in theresonator. Surface 98A of prism 98 is arranged such that lightcirculating in the resonator is incident at the Brewster angle thereon.

In resonator 134 of MOPO 130, and in resonator 114 of MOPO 110, lightcirculating in the resonators makes an even number of fold reflectionsper round trip in the resonator. There are six fold reflections forresonator 114 and four-fold reflection for resonator 134. In a ringresonator for a MOPO in accordance with the present invention, an evennumber of reflections per round trip is preferred as this provides thatthe resonator stays in alignment for slight variations in input angle ofseed beam 28. It is this preference for an even number of reflections inresonator 114 that occasions the use of two fold mirrors 120 and 122instead of a single mirror for making a 90° fold in the beam path.

The above-discussed problem of cross talk between an amplifier and anoscillator is virtually eliminated by employing a ring resonator aspower oscillator. In every ring resonator, mirrors forming the ringresonator are aligned at non-normal incidence angle to the optical axisof the resonator. Accordingly, no element in the resonator is aligned atnormal incidence to the optical axis of the master oscillator and is ina position to direct radiation back into the master oscillator. Further,a ring oscillator works as a unidirectional amplifier. By way ofexample, in ring resonator 134 of FIG. 14, light circulatescounter-clockwise through the ring. In order to reach master oscillator24, the light would have to travel clockwise. This only occurs for ASE,which is of much lower intensity than the laser beam.

An above-discussed advantage of the ring resonator power oscillator isan ability to generate a longer pulse than prior-art power oscillatorsemploying a short standing-wave resonator. It is also possible togenerate a longer pulse using a long standing wave resonator, however,in order to do this while generating a high power sufficient for apractical deep ultraviolet (DUV) lithography system, some measures mustbe devised to avoid optical damage to resonator components, inparticular the output mirror as discussed above. By way of example,optical components are easily damaged at the laser fluence levelsexceeding roughly 100 millijoules per square centimeter (mJ/cm²).

While above described embodiments of inventive ring-resonator poweroscillators are capable of generating a longer pulse than prior-artstanding-wave power oscillators, it has been determined that a longerpulse can be generated by an oscillator having a standing-wave resonatorthat is significantly longer that that of a prior-art power oscillator.By way of example, a prior-art linear resonator for an excimer laser hasa length of 1.5 meter or less, in order to reduce the time necessary forpulse evolution. In a power oscillator in accordance with the presentinvention a resonator length of about 2 meters or greater is preferred.A description of principal advantages of such a long resonator is setforth below with reference to FIG. 15.

FIG. 15 schematically illustrates an extended linear resonator having alength L, formed between a rear mirror M₁ and outcoupling mirror M₂. Again medium is located at a distance L₁ from rear mirror M₁, and at adistance L₂ from outcoupling mirror M₂. Seed beam 28 (seed pulse) isinjected into the resonator via mirror M₁. The first pass of the pulsethrough the gain medium occurs at the time instance t₁, while the secondand third passes occur at time instances t₂ and t₃ respectively. Since aportion of the light is output through outcoupling mirror M₂ on eachround-trip, what is seen at the output are at least two pulses delayedwith respect to each other by a time interval (t₃-t₁). This timeinterval is equal to L/c, where c is the speed of light, and representsthe total round-trip time of light in the resonator. Accordingly, thelonger the resonator the longer the output pulse that can be achieved.It is important, however, that the time interval between the first twopasses (t₂-t₁) does not exceed the duration of a gain pulse (pulsedelectrical discharge) in the gain medium. The time interval (t₂-t₁)depends on the length L₂, and not the total length L. Because of this,time interval (t₂-t₁) can be reduced by placing the gain medium closerto outcoupling mirror M₂ than to rear mirror M₁. Because of this, totalresonator length L can be increased in order to achieve a longer outputpulse, without a penalty of lower gain per round-trip.

An additional advantage of an increased resonator length L is thereduced level of ASE emitted from outcoupling mirror M₂ together withlaser (amplified pulse) output. A reason for this, as discussed abovewith respect to embodiments of inventive ring power oscillators, is thatASE has a large divergence angle compared with laser radiation. Becauseof this, as ASE propagates from the gain medium towards rear mirror M₁and then back towards the gain medium, the proportion of ASE to laserradiation is effectively attenuated due to the difference in divergence.

For comparison of the long, linear resonator with the above-describedring resonators, FIG. 16 schematically represents a generic ringresonator formed by mirrors, M₃, M₄, M₅, and M₆. Here, the totalround-trip time cannot be increased without the penalty of increasingtime interval (t₃-t₁) between the passes in the gain medium. In the longlinear resonator of FIG. 15, a practical limit of increasing totallength of resonator L and reducing L₂ in the is set by considerations asfollows. If rear mirror M₁ is placed too close to the master oscillator(not shown in FIG. 15) feedback to the oscillator will be increased. Thecloser the gain module is placed to mirror M₂, the more ASE is generatedin a direction towards the master oscillator, i.e., towards mirror M₁.Space for the long resonator in a MOPO may be limited.

FIGS. 17A and 17B schematically illustrate an eleventh embodiment 140 ofa MOPO in accordance with the present invention including a masteroscillator 24 including line-narrowing elements 25 (not shown in FIG.17B) and a bandwidth-limited power oscillator 142 including a longstanding-wave resonator 144. Resonator 144 includes again a chamber 60.Gain chamber 60 includes spaced-apart parallel electrodes 65A and 65Bfor exciting a discharge in lasing gas in the gain chamber. FIG. 17A isa view seen along an axis generally referred to as the “long beam axis”of BLPO 142, perpendicular to the plane of the electrodes, being theaxis in which an amplified beam in gain medium is widest. FIG. 17B is aview seen along the “short beam axis” of the BLPO, perpendicular to thelong beam axis. The long and short beam axes are depicted in FIGS. 17Aand 17B as being the X-axis and the Y-axis, respectively, with theZ-axis being the longitudinal axis of the BLPO. The Z-axis is folded byvarious resonator components.

Resonator 144 is a long, stable, linear resonator formed between a rearmirror 56 and an outcoupling mirror 58. Seed beam 28 from masteroscillator 24 is reflected by mirrors 30 and 32 into resonator 144 viamirror 56. Rear mirror 56 is a partially transmitting mirror, forexample, having a transmission between about 5% and 20%. This allowscoupling of a portion of the beam 28 into resonator 44 while reflectingmost of the light generated in the resonator. An optimal value oftransmission for mirror 56 is determined by a trade-off between couplingin more light from the master oscillator, and transmitting less light(back out of the mirror) from BLPO 142. An (optional) isoscelestriangular prism 62 is included in resonator 144 for providingadditional ASE suppression over that which is characteristic of theinventive long (2 meters or greater) resonator. This prism is notnecessary for obtaining beam expansion advantages of the resonator andmay be omitted if the additional ASE suppression is not needed. Itshould be noted that without prism 62 or any similar bandwidth-limitingelement, power oscillator 142 could not be characterized as abandwidth-limited power oscillator.

Resonator 144 includes a convex cylindrical mirror 148 and a concavecylindrical mirror 150, details of the arrangement of the mirrors aredepicted in FIGS. 18A (long beam axis view) and 18B (short beam axisview). Mirror 148 has a focal length f₁. The cylinder axis of mirrors148 and 150 (dotted line 152 in FIG. 18B) is in a plane containing theresonator axis. Mirror 148 expands a collimated beam 158, from gainchamber 60, in the short axis (see FIG. 18B), while the beam size in thelong axis remains unchanged (see FIG. 18A). Mirror 150 has a focallength f₂ greater than focal length f₁ of mirror 148. The cylinder axisof mirror 150 is parallel to that of the mirror 148. The mirrors arearranged parallel to each other and parallel to a common focal plane154. Because of this, mirror 150 re-collimates beam 158 in the shortaxis. Mirrors 148 and 150 constitute what can be described as agrazing-incidence (cylindrical) telescope.

Both mirrors 148 and 150 are arranged at a grazing incidence, i.e., atan incidence angle greater than 60°, to the incoming beam, for example,at an angle of 75°. This arrangement reduces the power density at thesurface of mirror 148 by a substantial factor, for example, by factor offour at 75° incidence. This substantially avoids damage, and reduces thedegradation rate of the mirror. After passing through the grazingincidence telescope, the beam is sufficiently expanded in the short axisto avoid damage to outcoupling mirror 58 and to any optical elementsdownstream of BLPO 142. Essentially, the expansion ratio of the beam inthe short axis is equal to the ratio of the magnitudes of the focallengths of mirrors 148 and 152. In one example of BLPO 142, an exemplaryexpansion ration of 4:1 for beam 158 in an arrangement with 50 W outputat 4 kilohertz (kHz), provides an expanded (output) beam size of 12 mmby 6 mm. This reduces laser fluence at outcoupling mirror 58 to 17mJ/cm², which is within a safe range as far as optical damage isconcerned.

An alternative grazing-incidence mirror arrangement for mirrors 148 and150 would be to make both mirrors concave, and increase the distancebetween the mirrors to maintain a confocal arrangement. In such anarrangement, beam 158 would come to an intermediate focus between themirrors in the short axis. Those skilled in the art may devise othergrazing incidence telescope or beam expander arrangements withoutdeparting from the spirit and scope of the present invention.

One advantage of using the inventive grazing-incidence telescope beamexpander over a prism beam expander is that, in the prism beam expander,the incidence angle of light on the prism surface is preferably at orclose to the Brewster angle, in order to reduce reflection losses. Thisincidence angle provides an expansion ratio at the prism surface ofabout 1.7:1, which is insufficient to optimally reduce degradation anddamage. Another advantage of the inventive grazing incidence telescopearrangement is that grazing incidence mirrors of the telescope aredesigned for s-polarization of the incident light. This provides thatoptical losses in the mirrors and optical damage to the mirrors areoptimized.

FIGS. 19A and 19B schematically illustrate a twelfth embodiment 160 of aMOPO in accordance with the present invention having a long-resonatorBLPO oscillator 162 including a grazing incidence telescope beamexpander. FIGS. 19A and 19B are respectively long beam axis and shortbeam axis views. MOPO 160 is similar to MOPO 140 but differs in thearrangement of the grazing incidence telescope beam expander. In BLPO162, the beam expander comprises above-described grazing incidencecylindrical mirrors 148 and 150 arranged with a plane mirror 166 (alsoat grazing incidence) therebetween. Here, mirrors 148, 150, and 166 foldthe resonator axis by 90° in order to provide the desired resonatoraxial (optical) length while shortening the physical “footprint” of theresonator.

Folding the inventive long resonator is not limited to folding withelements of the grazing incidence telescope. It is also possible fold along resonator power oscillator using prisms. By way of example, twotriangular prisms 62 may be included in the resonator and arranged tofold the resonator by 90° as in resonator 52 of FIG. 3. A long resonatormay also be folded by 90° using one Pellin-Broca prism or by 180° usingtwo Pellin-Broca prisms. These and other folding arrangements may beused without departing from the spirit and scope of the presentinvention.

In the description of the MOPO lasers of FIGS. 17A-B and FIGS. 19A-B, itis assumed, for convenience of description, that a beam leaving gainchamber 60 is collimated and is expanded and re-collimated by agrazing-incidence telescope. In practice, as discussed above, the beamhas a natural divergence, which means that the size of the beam expandswith distance traveled, and it is preferable to compensate for thedivergence. Within one round trip of the beam in a standing waveresonator, the beam has to be compressed to its original size. Inabove-discussed power oscillators having ring resonators, prism elementsare used for compensating beam divergence. Prisms cannot be used in alinear resonator for compensating the divergence, since for one completeround trip a prism does not have any optical power.

In the resonators of FIGS. 17A-B and FIGS. 19A-B, the grazing-incidencecylindrical mirrors 148 and 150 can conveniently be used forcompensating the beam divergence in the short beam axis. This can beachieved by slightly changing the spacing of the grazing-incidencecylindrical mirrors so that they are no longer confocally arranged. Aspacing between the two telescope mirrors can be found where the opticalpower of the grazing incidence telescope matches the natural divergenceof the beam and minimizes the effective divergence of the beam insidethe resonator.

As noted above, in the resonators of FIGS. 17A-B and 19A-B, an optionalprism 62 is provided for providing additional ASE suppression bybandwidth limiting, i.e., spectrally diverting ASE wavelengths from theresonator axis. It is almost always preferable to suppress as muchbackground as possible due to ASE. Each of the resonators acts, to acertain degree, as a spatial filter. The longer the resonator, the morethe ASE beam is expanded before it re-enters the gain medium, and theless is the ASE that enters gain chamber 60 and is amplified. Because ofthis, a long linear resonator reduces the ASE level more effectivelythan a short linear resonator.

In the resonators of FIGS. 17A-B and 19A-B, this applies to ASE that isemitted originally towards rear mirror 56, and then passes gain chamber60 before being output through outcoupling mirror 58. However, ASEemitted in the opposite direction may not be attenuated to the samedegree, due to a shorter distance between the gain chamber and the rearmirror. Accordingly, this consideration sets the above-discussedpractical upper limit to the ratio L₁/L₂ (see FIG. 15) at a given totallength L. In the resonators of FIGS. 17A-B, one isosceles triangularprism 62 is provided for further reduction of ASE. It should be noted,however, that other wavelength-dispersive devices may be included in theresonators, including more than one prism, or one or more diffractiongratings, or optical etalons, without departing from the spirit andscope of the present invention.

FIGS. 20 and 21 are short beam axis views schematically illustratingthirteenth and fourteenth preferred embodiments 180 and 194 respectivelyof a MOPO in accordance with the present invention. These inventiveMOPOs are similar to the MOPOs of FIGS. 17A-B and 19A-B, respectively,with an exception that a different arrangement is provided for injectinga seed beam (pulse) 28 into the long-resonator power oscillator, andwith an exception that optional prism 62 has been omitted. In FIGS. 20and 21, a master oscillator is not shown.

MOPO 180 has a power oscillator 182 having a linear resonator 184.Resonator 184 is formed between an outcoupling mirror 58 and a thin filmreflector (mirror) 186 deposited on a surface 192A of a prism 192. Prism192 is configured and arranged in the resonator such that adjacent face192B thereof is at a non-normal incidence angle to a circulating beam,i.e., at an angle to the resonator axis, and such that the beam isnormally incident on face 192A, i.e., on mirror 186. Seed beam 28 isincident on prism 192 at an angle such that it is directed into gainchamber 60 along the path of the circulating beam, i.e., the seed beamis incident on the prism at an equal but opposite angle to the angle ofincidence of the circulating beam. MOPO 194 has a power oscillator 196having a linear resonator 198. Resonator 198 is also formed between anoutcoupling mirror 58 and a thin film reflector (mirror) 186 depositedon a surface 192A of a prism 192 with seed beam 28 being incident onprism 192 at an angle such that it is directed into gain chamber 60along the path of the circulating beam.

An advantage of the prism injection arrangement of MOPOs 180 and 194 fora seed pulse, compared with the rear mirror seed-pulse injectionarrangement of MOPOs 160 and 170, is that the possibility of feedback tothe master oscillator is essentially eliminated. A disadvantage is thatthe circulating beam in the resonator traverses prism 192 twice perround-trip, and, as a result of this, suffers double optical losses,particularly from reflections at surface 192B, which must be sufficientto ensure adequate beam injection. Those skilled in the art willrecognize that a separate, dielectric-coated beamsplitter tilted to theresonator axis could be used for seed beam injection, together with aconventional plane rear mirror, instead of the prism arrangement ofFIGS. 20 and 21. However, this may not be as optically efficient as themirrored prism arrangement. It should be noted that the arrangement ofprism 92 could function simultaneously as a wavelength-dispersiveelement similar to prisms in certain above-discussed embodiments of thepresent invention. In order for this to occur, however, the prismarrangement would need to be oriented at 90° to the orientation depictedin FIGS. 20 and 21.

It should be noted, here, that the grazing-incidence telescopearrangement comprising cylindrical mirrors 148 and 150 may be deployedoutside of the resonator of a power oscillator for expanding an outputbeam delivered by that power oscillator before the beam is delivered toother optical elements. FIG. 22A and FIG. 22B are, respectively, longbeam axis and short beam axis views schematically illustrating a system200 including a master oscillator 24 having line narrowing elements 25and a generic power oscillator 202. Power oscillator 202 may be aring-resonator or a linear resonator including inventive elementsdescribed above, or may be a prior-art power oscillator or even aprior-art power amplifier. The master oscillator delivers a seed beam 28to the power oscillator as described above. Output radiation isdelivered from power amplifier 202 to a grazing incidence telescopecomprising cylindrical mirrors 148 and 150 configured and arranged asdescribed above with reference FIGS. 18A-B. The grazing incidencetelescope delivers a short-axis expanded beam.

In a MOPO laser system, the optical component, other than theoutcoupling device, that usually sees the high laser radiation powerdensity is a window (usually at Brewster's angle to the beam), in thegain chamber of the resonator of the power oscillator. Inabove-described embodiments of the present invention the gain chamber isgain chamber 60, and chamber windows are not shown for convenience ofillustration. In a practical example of a power oscillator in accordancewith the present invention a window in the gain chamber closest to theoutcoupling mirror (or prism) will receive a laser fluence of betweenabout 50 mJ/cm² and 100 mJ/cm². This can lead to a problem of shortlifetime of the window of the gain chamber. A description of opticalarrangements for mitigating this window lifetime problem is set forthbelow with reference first to FIG. 23.

FIG. 23 is a long beam-axis view schematically illustrating a sixteenthembodiment 206 of a MOPO laser in accordance with the present invention.MOPO 206 has a master oscillator 24 including line narrowing elements25, and a power oscillator 208. Power oscillator 208 has aring-resonator 115 with optical elements thereof arranged similarly tothose of resonator 114 of power oscillator 112 of FIG. 13. Surface 126Aof a prism 126 serves as a beamsplitter for coupling a portion ofcirculating laser radiation out of the resonator as in theabove-described resonator. The circulating beam is incident at grazingincidence on surface 126A for expanding the beam and thereby reducingoptical fluence on the surface.

In resonator 115 of FIG. 23, prism 126 is located in a gain chamber 61.Gain chamber 61 has a section 61A thereof including discharge electrodes(only lower electrode 65B shown) and a portion 61B thereof includingprism 126. A front Brewster angle window 67 is located in section 61A ofgain chamber 61. In section 61B, a window 69 is arranged at the Brewsterangle to the beam circulating in the resonator, and a window 71 isarranged at the Brewster angle to the output beam after the beam hastraversed prism 126 and exited the prism via face 126 thereof.

In resonator 115, the laser fluence on surface 126 is much lower thanwould be the fluence on a prior-art Brewster angle window in a gainchamber. By way of example, if the grazing incidence angle on surface126A is 82°, the laser fluence will be about 4.0 times lower than itwould be on a Brewster-angle surface. If prism 126 is a CaF₂ prism, andthe circulating beam has a wavelength of 193 nm (an ArF excimer laserwavelength), Fresnel reflection at surface 126A for p-polarized light isabout 30%, and the transmitted (output) portion is expanded to about 5.4times the width of the circulating beam. This beam expansion, asdiscussed above, provides that surface 126B of the prism can be atnormal incidence or at near normal incidence to the output beam. A smallincidence angle from normal, for example about 3.7°, at the surface 126Bof prism 126 can reduce the probability of reflecting radiation backinto the gain area (not explicitly shown) of gain chamber 61. Providingan antireflection coating on surface 126B can also be helpful in thisregard.

It should be noted, here, that the gain chamber arrangement of chamber61 is not limited to use with ring resonator 114 but can be used inother power oscillator ring resonators or in power oscillator linearresonators, whether or not these resonators include any line-narrowingelements or beam-expanding devices in accordance with the presentinvention. By way of example, FIG. 24 schematically illustrates a poweroscillator 210 having a linear resonator 212 including a gain chamber61. Resonator 212 is formed between mirror M₁ and M₂ and is intended torepresent simply any “generic” linear resonator. A seed beam 28 isreflected from mirror 32 into resonator 212 via mirror M₁ thereof.Although gain chamber 61 of FIG. 24 is somewhat different in shape fromgain chamber 61 of FIG. 23, and has a different outcoupling device, thesame reference numeral is used for the chamber and common componentsthereof. In gain chamber 61 of FIG. 24, a prism 214 functions as anoutcoupling device. Surface 214A of the prism is at grazing incidence tothe circulating beam. Prism angles are arranged such that an output beamtraversing the prism exits the prism at the Brewster angle to surface214B thereof. This eliminates any possibility for back reflection ofradiation into the gain chamber and any need for an antireflectioncoating on surface 126B. Windows 69 and 71 are arranged at the Brewsterangle to the circulating beam and the output beam, respectively.

In summary, the present invention is described above with reference to apreferred and other embodiments. The invention, however, is not limitedto the embodiments described and depicted. Rather, the invention islimited only by the claims appended hereto.

1. A laser system, comprising: a master oscillator for generating a beamof laser radiation, said master oscillator including line narrowingelements for limiting the bandwidth of the beam of radiation; and apower oscillator arranged to receive and amplify said laser radiationbeam, said power oscillator having optical elements defining aresonator, said resonator including a gain medium and wherein said poweroscillator further includes at least one prism arranged for expandingthe beam for reducing the power density on the optical elements of theresonator in order to reduce optical damage.
 2. A laser system asrecited in claim 1, wherein the resonator of the power oscillator is aring resonator.
 3. A laser system as recited in claim 1, wherein theresonator of the power oscillator is a linear resonator.
 4. A lasersystem as recited in claim 1, wherein the resonator of the poweroscillator includes an input coupler for coupling radiation from themaster oscillator into the power oscillator.
 5. A laser system asrecited in claim 4, wherein said input coupler also functions as anoutput coupler.
 6. A laser system as recited in claim 5, wherein theresonator of the power oscillator is a ring resonator.
 7. A laser systemas recited in claim 1, wherein a pair of prisms are used to perform thebeam expanding function.
 8. A laser system, comprising: a masteroscillator for generating a beam of laser radiation, said masteroscillator including line narrowing elements for limiting the bandwidthof the beam of radiation; and a power oscillator arranged to receive andamplify said laser radiation beam, said power oscillator having opticalelements defining a ring resonator, said resonator including a gainmedium and wherein said power oscillator further includes a pair ofprisms arranged for expanding the beam for reducing the power density onthe optical elements of the resonator in order to reduce optical damage.9. A laser system as recited in claim 8, wherein the resonator of thepower oscillator includes an input coupler for coupling radiation fromthe master oscillator into the power oscillator.
 10. A laser system asrecited in claim 9, wherein said input coupler also functions as anoutput coupler.
 11. A laser system, comprising: a master oscillator forgenerating a beam of laser radiation, said master oscillator includingline narrowing elements for limiting the bandwidth of the beam ofradiation; and a power oscillator arranged to receive and amplify saidlaser radiation beam, said power oscillator having optical elementsdefining a resonator, said resonator including a gain medium and whereinsaid power oscillator further includes means for expanding the beam forreducing the power density on the optical elements of the resonator inorder to reduce optical damage.
 12. A laser system as recited in claim11, wherein said means for expanding the beam includes at least onegrazing incidence mirror.
 13. A laser system as recited in claim 11,wherein said means for expanding the beam includes a pair of prisms. 14.A laser system as recited in claim 11, wherein the resonator of thepower oscillator is a ring resonator.
 15. A laser system as recited inclaim 11, wherein the resonator of the power oscillator is a linearresonator.
 16. A laser system as recited in claim 11, wherein theresonator of the power oscillator includes an input coupler for couplingradiation from the master oscillator into the power oscillator.
 17. Alaser system as recited in claim 16, wherein said input coupler alsofunctions as an output coupler.
 18. A laser system as recited in claim17, wherein the resonator of the power oscillator is a ring resonator.